Inter base station handover method, radio communication system, DRX control method, base station, and communication terminal

ABSTRACT

The present invention provides a DRX control method and system in which power consumption of a mobile station can be reduced and an increase in the load of a network can be suppressed. A source base station forwards Dormancy Context, which is information for controlling the activity level of a mobile station that performs inter base station handover, to a target base station and, immediately after the mobile station completes handover, the target base station performs DRX control of the mobile station using the Dormancy Context.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation application of U.S. application Ser.No. 12/525,950 filed Aug. 5, 2009, which claims priority from Japanesepatent application 2007-025873 filed on Feb. 5, 2007, the disclosures ofwhich are incorporated herein by reference in their entirety.

TECHNICAL FIELD

The present invention relates to a radio communication system, and moreparticularly to a radio communication system and method that performs aninter base station handover from a source base station to a target basestation.

BACKGROUND ART

In 3GPP (3rd Generation Partnership Project) Long Term Evolution (LTE),study is being conducted that the following information (RAN(RadioAccess Network) Context Data), which is the information concerning amobile station that performs inter base station handover, is transferredfrom a source base station (source eNB) to a target base station (targeteNB), when the mobile station performs inter base station handover(abbreviated “HO”). (For example, see non-Patent Document 1).

1. Qos profiles (QoS profiles of the SAE (System Architecture Evolution)bearers)

2. AS configuration (RLC(Radio Link Control) Window Size, etc.)

In addition, when the source base station is transmitting downlink data,the source base station performs data forwarding which transfers unsentdata to the target base station.

The mobile station that has moved to the target cell accesses the targetbase station via Random Access Channel (RACH), which is an uplinkchannel, to acquire Timing Advance (TA), provided for uplinksynchronization, and uplink scheduling information from the target basestation. After that, the mobile station adjusts transmission timingaccording to the acquired TA and transmits “HO Confirm”, which is acontrol signal for notifying that the mobile station has performedhandover to the target base station, at allocated time and frequency.

In LTE, study is being conducted also on DRX (Discontinuous Reception:intermittent reception) control of a mobile station in the RRC (RadioResource Control)_Connected state (see Non-Patent Document 1).

A base station performs DRX control of entire mobile stations in a cellthat the base station manages, and a mobile station performs thediscontinuous reception at a periodic interval (also called “DRX cycle”or “DRX period”) specified by the base station. A DRX cycle (DRX period)includes a reception period during which data is continuously receivedand a non-reception period during which no data is received, as shown inFIG. 18.

Non-Patent Document 1:

-   3GPP TS36.300 v0.3.1 (Section 10.1, etc.)    Non-Patent Document 2:-   3GPP RAN WG2[R2-070088 Summary of email discussion on DRX in    LTE_ACTIVE], <Internet URL    http://www.3gpp.org/ftp/tsg_ran/WG2_RL2/TSGR2_(—)56bis/Documents/R2-070088.zip>

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

The disclosure of Non-Patent Documents 1 and 2 given above is herebyincorporated by reference into this specification. The following givesan analysis of the technology related to the present invention.

Today, study has just begun on how to combine the LTE-proposed interbase station HO control and the DRX control. And so, little study hasbeen made on a practical method for power saving on a mobile stationwhen those controls are combined.

Therefore, in view of the problems described above, it is an object ofthe present invention to provide a method, system, base station, andcommunication terminal for saving power of a mobile station whenhandover control and DRX control are combined.

Therefore, it is another object of the present invention to provide amethod, system, base station, and communication terminal for enablingsuppression of an increase or reduction in a load of a network sideinvolved in handover control.

Means to Solve the Problems

To solve one or more the problems of the described above, the inventiondisclosed by this application provides an inter base station handover(HO) method and a radio communication system, which implements themethod, that have the following general configuration.

In an inter base station handover method and a radio communicationsystem of the present invention, a source base station (source eNB)forwards Dormancy Context to a target base station (target eBN) duringhandover to optimize the continuation of DRX control before and afterhandover.

After a mobile station has completed handover, the target base station(target eBN) uses the Dormancy Context to perform DRX control of themobile station.

If the mobile station (User Equipment: UE) has stayed in a long DRXcycle in the source cell, the target base station (target eBN) may usethe Dormancy Context also for the processing for moving the state of themobile station (UE) to LTE_Idle.

In the present invention, at least one of the following is included inthe Dormancy Context.

a DRX (Discontinuous Reception) level at the current time (when a HOrequest is generated),

a time during which a mobile station has stayed in the current DRXlevel,

an average DRX level during management by the source base station,

a maximum DRX level during management by the source base station,

a minimum DRX level during management by the source base station,

a transmission buffer size in the HO preparation period, and

a scheduling time in the source base station/RRC_Connected state time inthe source base station

A source base station and a target base station in the present inventionmay be base stations not only in the same communication system but alsoin different systems.

Effect of the Invention

According to the present invention, it is possible to optimize thecontinuation of DRX control before and after handover. For example, thepresent invention allows a low activity mobile station to start DRXcontrol more quickly. As a result, the reduction of the powerconsumption of the mobile station can be achieved.

The present invention also allows a low activity mobile station totransit to an Idle state more quickly. As a result, the reduction of thepower consumption of the mobile station can be achieved. In addition, inthe present invention, it is possible to avoid inter base station HOwhich is not actually necessary, thus avoiding an increase in the loadof a network.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram for explaining a flow of inter base station handoverin one exemplary embodiment of the present invention.

FIG. 2 is a diagram for explaining inter base station handover in oneexemplary embodiment of the present invention.

FIG. 3 is a diagram for explaining activity level control of a mobilestation after inter base station handover in one exemplary embodiment ofthe present invention.

FIGS. 4A and 4B are diagrams for explaining activity level control of amobile station after inter base station handover in one exemplaryembodiment of the present invention.

FIGS. 5A and 5B are diagrams for explaining activity level control of amobile station after inter base station handover in one exemplaryembodiment of the present invention.

FIG. 6 is a diagram for explaining activity level control of a mobilestation in a first exemplary embodiment of the present invention.

FIG. 7 is a diagram for explaining the effect of power saving of amobile station in the first exemplary embodiment of the presentinvention.

FIG. 8 is a diagram for explaining activity level control of a mobilestation in a second exemplary embodiment of the present invention.

FIG. 9 is a diagram for explaining the effect of power saving of amobile station and the effect of reduction in the load of a NW in thesecond exemplary embodiment of the present invention.

FIG. 10 is a diagram for explaining activity level control of a mobilestation in one exemplary embodiment of the present invention.

FIG. 11 is a diagram for explaining a flow of inter base stationhandover.

FIG. 12 is a diagram for explaining activity level control of a mobilestation in an example.

FIG. 13 is a diagram for explaining an operation of a mobile station inan example.

FIG. 14 is a diagram showing an example of the configuration of a basestation in one example of the present invention.

FIG. 15 is a diagram showing an example of the configuration of a mobilestation in one example of the present invention.

FIG. 16 is a diagram showing an example of the configuration of a mobilestation in another example of the present invention.

FIGS. 17A and 17B are diagrams for explaining HO in another example ofthe present invention.

FIG. 18 is a diagram for explaining a DRX cycle.

FIG. 19 is a diagram for explaining a flow of inter base stationhandover in a modification of one exemplary embodiment of the presentinvention.

EXPLANATIONS OF SYMBOLS

-   1,2 LTE base station-   4 Base station control station-   5 WCDMA base station-   101 Source base station-   102 Target base station-   3,103 Mobile station-   41,42 Transmission/reception interface-   43 Control unit-   44 Dormancy control relay unit-   104 MME/UPE-   105 Radio unit-   106 Baseband unit-   107,112 Coding/decoding unit-   108 Control unit-   108-1 Scheduler-   108-2 Controller-   109 Transmission/reception unit-   110 DRX controller-   111 Buffer-   121 Radio unit-   122 Baseband unit-   123,127 Coding/decoding unit-   124 Buffer-   125 Control unit-   126 Activity level controller-   127 DRX level controller

PREFERRED MODES FOR CARRYING OUT THE INVENTION

To describe the present invention more in detail, exemplary embodimentsof the present invention will be described below with reference to theaccompanying drawings. In the exemplary embodiment below, an example inwhich the present invention is applied to a system proposed by 3GPP LTEis used though not limited thereto.

As an example of the DRX (Discontinuous Reception) control at handover(abbreviated “HO”), a base station changes the parameters (see FIG. 18)related to a DRX cycle, such as the non-reception period, according tothe data transmission/reception status (called “Activity”) of a mobilestation. Although a base station changes the parameters in thedescription below because an example of 3GPP LTE is used, the networkside, for example, a 3GPP base station control apparatus (RNC: RadioNetwork Controller), may change the parameters. As an indicatorindicating an Activity rate (degree), the indicator such as an “Activitylevel” may be used. As this activity level, the ratio of the time (Ts),during which data is accumulated in the transmission buffer, to apredetermined period (T) may be used ((Ts/T)×100(%) when represented in%). Note that, in the present invention, the Activity level is notlimited to (Ts/T)×100(%) but that another value (conversion value)having a correlation with (Ts/T) may also be used.

Although a practical example of “Activity” and “Activity level” wasdescribed above, the definition of “Activity” and “Activity level” inthis specification is not of course limited to the above but should beunderstood as the general data transmission/reception status and itsfrequency.

A base station and a mobile station may use an indicator called “DRXlevel”, acquired based on the Activity level, as the signal used for theDRX control. An Activity level value may be used directly as a DRXlevel, or a value acquired by converting an Activity level value(preferably a value having a high correlation with an Activity levelvalue), may be used as a DRX level. A DRX level may be represented inpercent(%). In this case, though a discrete value (for example, aninteger value) is usually used as a value in the range of 0-100%, acontinuous value such as a decimal number may of course be used.Alternatively, several discrete central values may be used as DRXlevels.

First, as an example of the LTE-proposed operation in which the interbase station (inter eNB) HO and the DRX control are combined, thefollowing describes the DRX control of a mobile station in the HOpreparation period during which a sequence of the following operationsare performed.

-   -   The mobile station transmits the Measurement Report to the        source base station.    -   The source base station checks the Measurement Report to        determine which base station is a candidate for the target base        station, and the source base station and the target base station        exchange the handover-related information indicating whether the        target base station can accept the handover or not.

For example, a method is known that a mobile station, for which a longnon-reception period is configured by the DRX control, ignores thecurrently configured DRX control and transits to the Active operation(the state in which the mobile station can continuously receive thedownlink signal) and that a mobile station, for which a shortnon-reception period is configured, performs HO, while staying in thestate in which the short non-reception period is configured (forexample, see Non-Patent Document 2). However, a practical method forimplementing the method described above is not shown. The followingdescribes a practical implementation method.

The procedure of base station HO in a mobile station under DRX controlwill be described with reference to FIG. 11.

Uplink scheduling information (UL allocation) is transmitted from thetarget base station to the mobile station. The mobile station that willperform inter base station HO transmits a Measurement Report onneighboring cells of the source cell, in which the mobile stationresides, to the source base station.

The source base station transmits a signal (DRX Control Signaling),which instructs the mobile station to move from DRX (DiscontinuousReception) to the continuous reception operation (or to reduce thenon-reception period of the DRX cycle), to the mobile station and stopsthe DRX control of the mobile station. It should be noted that, thoughthe base station that receives the Measurement Report outputs the DRXcontrol stop signal to the mobile station in the sequence operationexample in FIG. 11, the base station that receives the MeasurementReport need not always output the DRX control stop signal to the mobilestation. For example, a rule may be predetermined to allow the mobilestation to stop the DRX operation itself.

The source base station transfers the RAN Context Data (Qos Profile, ASconfiguration) on the mobile station to the target base station.

After receiving the notification (Context Confirm), which indicates thatthe HO can be accepted, from the target base station, the source basestation transmits the command of HO start permission (HO Command) to themobile station.

After receiving the HO start command (HO Command) from the source basestation, the mobile station that has moved to the target cell transmitsan uplink synchronization (UL Synchronization) request via an RACH thatis an uplink channel, and acquires the transmission timing adjustmentvalue (Timing Advance: TA) and the uplink scheduling information (ULallocation) from the target base station.

After that, the mobile station transmits HO Confirm at the allocatedtime and frequency, by adjusting the transmission timing according tothe transmission timing adjustment value (TA) received from the targetbase station, to inform the target base station that the mobile stationhas performed handover.

The target base station, which has received the HO Confirm from themobile station, transmits a control signal (HO Completed) to the sourcebase station to inform that the handover has been completed, notifiesthe MME (Mobility Management Entity)/UPE (User Plane Entity) that themobile station performed the inter base station HO to move to the cellwhich is managed by the target base station (UE (User Equipment) updateto MME/UPE), and completes the inter base station HO operation. Notethat, at this point of time, the mobile station is still in the Activeoperation.

If the mobile station that has performed handover does not transmit orreceive data during a predetermined period (determined by the timerincluded in the target base station side) after handover, the targetbase station restarts DRX control for the mobile station.

The uplink scheduling information (UL allocation: time and frequencyallocation information) is transmitted from the target base station tothe mobile station to allow the mobile station to transmit data (UL datatransmission), if necessary.

As described above, the mobile station is controlled by combining HO andDRX.

FIG. 12 is a diagram for explaining the calculation of the time UEstaying in DRX shown in FIG. 13, and FIG. 12 shows an example of changesin the DRX level when a mobile station in DRX control operation performsthe inter base station HO. It is assumed that a mobile station movesthrough the center of each cell and moves the distance equal to thediameter.

In the example in FIG. 12, the state is called as follows.

-   -   “Active” when the DRX level is 100%, and    -   “DRX” when the DRX level is 20%.

An idle state in which the DRX level is 0% is RRC_Idle(LTE_Idle) state.

In the description below, a model proposed by 3GPP LTE is used as anexample where 100% of the DRX level is 100% of the Activity Level. Whenone frame includes 10 TTIs (Transmission Time Intervals), a mobilestation whose DRX level is 100% (that is, Active operation) monitors thedownlink (DL) signal (demodulates a control channel) in each TTI. On theother hand, a mobile station whose DRX level is 20% monitors the DLsignal only in the two continuous TTIs out of 10 TTIs but not in theremaining eight TTIs that are non-reception periods. It is of coursepossible to define a value of the DRX level smaller than 100%, forexample, 90% or 95%, as “Active”.

In the example in FIG. 12, cell 1 is managed by base station 1, andcells 2, 3, and 4 are managed respectively by base stations 2, 3, and 4.Also assume that the mobile station performs the datatransmission/reception only for the HO operation in cells 2, 3, and 4.The change in the DRX level is indicated by the bold line.

In FIG. 12, the symbol X indicates the time during which the mobilestation is Active during HO procedure and the symbol Y indicates thetime during which the mobile station is under DRX control (DRXresidence)

Now, assume that the mobile station is in cell 1 and is under DRXcontrol. When performing inter base station HO to move to cell 2, themobile station becomes Active and performs the HO operation. Because, incell 2, the mobile station does not perform data transmission/receptionimmediately after the HO, the base station 2 changes the state of themobile station from the Active to the DRX after a timeout of a timer,corresponding to the Active-to-DRX transition time, occurs. After that,the mobile station performs HO from cell 2 to cell 3, to cell 4, andthen to cell 5, one after another, and, in that case, the operation isperformed in the same way as when the mobile station performs HO fromcell 1 to cell 2.

Assume that the mobile station transition time from Active to DRX at HOis one minute (X in FIG. 12 is one minute) and that the mobile stationperforms data transmission/reception intermittently for 30 minutes.Under this assumption, FIG. 13 shows in a tabular form an example of theDRX residence time in 30 minutes when the parameters, such as themovement speed and the cell diameter, are varied.

In this example, the time required for HO, which will be several 10milliseconds, is so much shorter than the cell residence time that thistime is ignored in the calculation.

In FIG. 13, it is assumed that the frequency of the datatransmission/reception of the mobile station is not so high that themobile station can stay in DRX (the mobile station need not be kept inActive). In FIG. 12 and FIG. 13, a model is assumed in which cells 1 to5 are adjacent each other and the mobile station moves on a straightline at a constant velocity along the diameters of the multiple cells.

FIG. 13 shows that, when the movement velocity of the mobile station is120 km/h (2 km/minute) and the radius of the cell is 6 km (celldiameter=12 km),

the time the mobile station resides in each cell is 12 km/2 km=6 minutes(X+Y=6 minutes in FIG. 12), and

HO is performed 4 times.

Therefore, in 30 minutes, the mobile station moves across 5(=4+1) cellsand resides in the DRX cycle for 5(=6-1) minutes in each cell (that is,X=1 minute and Y=5 minutes in FIG. 12).

As a result, in 30 minutes, the mobile station stays in DRX for 5×5=25minutes.

On the other hand, when the movement velocity is 60 km/h and the celldiameter is 1 km, the residence time in each cell and the transitiontime from Active to DRX are both 1 minute and, therefore, the mobilestation does not transit to DRX for 30 minutes.

That is, in FIG. 12, Y=0 minute because X=1 minute and X+Y=1 minute,meaning that the next HO operation is started without starting the DRXcontrol in a target cell and, as a result, while the mobile stationmoves from cell 1 through to cell 5, HO is performed four times withoutperforming the DRX control.

As described above, when the residence time of a low-Activity mobilestation in one cell is shorter than the transition time to DRX (thistime is managed, for example, by a timer in the base station side), themobile station cannot make a transition to DRX in the cell and, as aresult, consumes extra power.

Similarly, when the residence time in a cell is shorter than thetransition time to the RRC_Idle state, the mobile station cannot make atransition to the RRC_Idle state and, as a result, consumes extra power.In this case, a mobile station, whose Activity is low enough to transitthe mobile station to the RRC_Idle state, repeats unnecessary HO. Suchwasteful HO will result in a higher network load (base station, UPE/MME)than it is supposed to be, and hence there is room for improvement.

An exemplary embodiment in another aspect of the present invention isthat the DRX control of a mobile station, which is performed in a targetcell after inter base station HO, is started at the same time the HO iscompleted. By doing so, the exemplary embodiment prevents the mobilestation from consuming extra power in the target cell and avoids therepetition of unnecessary HO, thus reducing the network load. In theexemplary embodiment described below, too, an example in which thepresent invention is applied to a system, proposed by 3GPP LTE, isdescribed though not limited thereto.

FIG. 1 and FIG. 2 are diagrams showing the flow (sequence diagram) ofinter base station HO of a mobile station, for which the DRX operationis performed in this exemplary embodiment, and the concept of the systemconfiguration.

A source base station (101) transmits uplink scheduling information (ULallocation) to a mobile station (103) and, before performing inter basestation HO, the mobile station (103) first transmits a MeasurementReport on neighboring cells of the source cell, where the mobile station(103) is now located, to the source base station (101).

The source base station (101) transmits the signal (DRX ControlSignaling) to the mobile station (103) to instruct the mobile station(103) to transit from the DRX to Active, and stops the DRX control ofthe mobile station (103).

The source base station (101) transfers Dormancy Context as well as QoSProfile and AS Configuration of the mobile station (101) to a targetbase station (102).

After receiving the notification signal (Context Confirm), whichindicates that the target base station (102) is ready to accept HO, fromthe target base station (102), the source base station (101) transmitsthe signal of HO start permission (HO Command) to the mobile station(101).

After receiving the control signal (HO Command) from the source basestation (101), the mobile station (103) accesses the target base station(102) via the RACH(Random Access Channel), which is a uplink channel, toacquire the transmission timing adjustment value (Timing Advance: TA)and uplink scheduling information (UL Allocation) from the target basestation (102).

The mobile station (103) adjusts the transmission timing according tothe transmission timing adjustment value (TA) and transmits the signal(HO Confirm) to the target base station (102) at the allocated time andfrequency to notify the target base station (102) that the mobilestation (103) has performed hand over.

The target base station (102) transmits the control signal (HOCompleted) to the source base station (101) and notifies an MME/UPE(104)that the mobile station (103), which performed the inter base stationHO, has moved to the cell managed by the target base station (102) (UEupdate to MME/UPE) and, after that, completes the inter base station HOoperation.

After the HO operation is completed, the target base station (102) usesat least Dormancy Context, which is one of the following that includeinformation on the mobile station in the source cell and that has beentransferred from the source base station (101),

QoS profile;

AS Configuration;

Dormancy Context;

Amount of packets arrived from UPE (User Plane Entity); and

Internal information that the target base station (102) has; to performthe DRX control of the mobile station (101) and transmits the signal(Early DRX Control Signaling) that makes the mobile station transit toan appropriate DRX state.

Any of the following may be used as an item for Dormancy Context.

(A) Current DRX level;

(B) Residence time in the current DRX level

(C) Average DRX level in the source cell

(D) Maximum DRX level in the source cell

(E) Minimum DRX level in the source cell

(F) Transmission buffer size in the HO preparation period

(G) Scheduling time in the source cell/RRC_Connected residence time inthe source cell

Although the DRX cycle (DRX period) is defined according to a DRX levelin this exemplary embodiment, the length of the DRX cycle may bedetermined according to a DRX level each time the base station performsthe DRX control operation. Alternatively, a table containing thecorrespondence between DRX levels and DRX cycles (DRX periods) may beprovided in a base station or a mobile station to determine the DRXcycle (DRX period) by referencing the correspondence table. It isdesirable that, for a higher DRX level, the length of the non-receptionperiod in the DRX cycle be lower than the length of the reception periodin the DRX cycle. This correspondence is assumed in the description ofthe exemplary embodiment below but the exemplary embodiment is notlimited to this setting.

The present invention provides the following methods for performing theDRX control.

(I) Fix the DRX cycle (DRX period) and adjust the ratio between thereception period and the non-reception period.

(II) Fix the reception period, and adjust the non-reception period. Atthe same time, vary the length of the DRX cycle (DRX period).

(III) Fix the ratio between the reception period and the non-receptionperiod, and adjust the DRX cycle (DRX period).

For each item for Dormancy Context, the following describes how todetermine L_(NEW) that is the DRX level of a mobile station in thetarget cell after the inter base station HO is performed.

(A) When the current (in the source cell at HO requested) DRX level(=L_(OLD)) is used as Dormancy Context, L_(NEW) is determined fromexpression (1) (see FIG. 3).L _(NEW) =L _(OLD) +M  (1)

where M is the predefined margin that is a fixed value. In the exampleshown in FIG. 3, M=25% and, because L_(OLD) is 25%, L_(NEW)=50%.

(B) When the current DRX residence time T and the current DRX level areused as the Dormancy Context, L_(NEW) is determined from expressions (2)and (3) (see FIGS. 4A and 4B).L _(NEW) =L _(OLD) +M ₂  (2)

$\begin{matrix}{M_{T} = \left\{ \begin{matrix}{M\; 1} & \left( {T \geq T_{0}} \right) \\{M\; 2} & \left( {T < T_{0}} \right)\end{matrix} \right.} & (3)\end{matrix}$

where M1 and M2 are predefined margins and M1<M2. T₀ is a threshold forselecting one of the predefined margins.

If the current DRX residence time T is greater than or equal to thethreshold time T₀, the margin M_(T) is set to M1; if T is less than T₀,M_(T) is set to M2. L_(NEW) is the value generated by adding M_(T) toL_(OLD).

(C) When the average DRX level (=L_(AVE)) in the source cell is used asDormancy Context, L_(NEW) is determined from expression (4).L _(NEW) =L _(AVE) +M _(AVE)  (4)

In this case, if an integer value is used as the DRX level, L_(AVE) isan integer greater than or equal to (or less than or equal to)(Σ_(i-1) ^(I) t _(i) L _(i))/Σ_(i-1) ^(I) t _(i)  (5)and is closest to that value, and M_(AVE) is the predefined fixedmargin.

(D) When the maximum DRX level (=L_(MAX)) in the source cell is used asDormancy Context, L_(NEW) is determined from expression (6).L _(NEW) =L _(MAX) +M _(MAX)  (6)

where M_(MAX) is a predefined margin that is fixed.

(E) When the minimum DRX level (=L_(MIN)) in the source cell is used asDormancy Context, L_(NEW) is determined from expression (7).L _(NEW) L _(MIN) +M _(MIN)  (7)

where M_(MIN) is a predefined margin that is fixed.

(F) When the transmission buffer size (S_(BUF)) of the source basestation in the HO preparation period in the source cell is used asDormancy Context, L_(NEW) is determined based on the relation between Kthresholds and K-1 DRX levels defined in advance as shown in expression(8) (see FIGS. 5A and 5B).

$\begin{matrix}{L_{NEW} = \left\{ \begin{matrix}{L_{K},\left( {S_{K} < S_{BUF} \leq \infty} \right)} \\\vdots \\{L_{2},\left( {0 < S_{BUF} \leq S_{1}} \right)} \\{L_{1},\left( {S_{BUF} = 0} \right)}\end{matrix} \right.} & (8)\end{matrix}$

In the example in FIG. 5A, because the transmission buffer size S_(BUF)of the source base station in the HO preparation period in the sourcecell is S₁≦S_(BUF)≦S₂, when L_(OLD) is 25%, we haveL _(NEW)=50%

from the table of correspondence between buffer thresholds and L_(NEW)shown in FIG. 5B. The table of correspondence between buffer thresholdsand L_(NEW) is held in a memory (for example, rewritable nonvolatilememory) that can be referenced by the controller in the base station.

(G) When the scheduling time in the source cell/RRC_Connected residencetime in the source cell (R_(SCR)) is used as Dormancy Context, L_(NEW)is determined based on the relation between K thresholds and K-1 DRXlevels defined in advance as shown in expression (9).

$\begin{matrix}{L_{NEW} = \left\{ \begin{matrix}{L_{K},\left( {R_{K} < R_{SCR} \leq \infty} \right)} \\\vdots \\{L_{2},\left( {R_{1} < R_{SCR} \leq R_{2}} \right)} \\{L_{1},\left( {0 < R_{SCR} \leq R_{1}} \right)}\end{matrix} \right.} & (9)\end{matrix}$

(H) When two or more (J) items described above are used as DormancyContext, L_(NEW) is determined from expression (10).L _(NEW)=Σ_(j-1) ^(J) w _(j) ·L _(NEW,j) +M  (10)

where w_(j) is the weight on L_(NEW,J), determined from the jth DormancyContext, and satisfies the following relation.Σ_(j-1) ^(J) w _(j)=1  (11)

The following describes some examples.

First Exemplary Embodiment

FIG. 6 and FIG. 7 are diagrams showing a first exemplary embodiment ofthe present invention. In the first exemplary embodiment, the currentDRX cycle is used as Dormancy Context, and the DRX level of the mobilestation in the HO completion period in the target cell is determinedusing expression (12) shown below (same as expression (1) shown above).L _(NEW) =L _(OLD) +M  (12)

In this example, M=0, that is, the DRX level in the target cell in theHO completion period is set to the same state as that of the DRX levelin the source cell immediately before the HO operation was started.

In this example, the DRX level of the mobile station is called asfollows.

“Active” when the DRX level is 100%

“Short DRX” when the DRX level is 60%

“Long DRX” when the DRX level is 20%

“Idle” when the DRX level is 0%

As described above, a DRX level lower than 100%, for example 90%, may bedefined as “Active”. For example, the ratio of the non-reception periodof the “Short DRX” in the DRX cycle is configured shorter than thenon-reception period of “Long DRX”.

Assume that there are four cells, 1, 2, 3, and 4, and that the cells andbase stations are related in such a way that cells 1, 2, 3, and 4 aremanaged respectively by base stations 1, 2, 3, and 4.

Also assume that the mobile station performs HO in cells 1, 2, 3, and 4in this order and that, in cells 2, 3, and 4, the mobile stationperforms data transmission/reception only for the HO operation.

Assume that the change in the DRX level in the present invention is asshown in FIG. 6 when the mobile station, which is under control of theDRX operation, performs inter base station HO.

In the initial state, assume that the mobile station that is moving fastresides in cell 1 and that the DRX level is Long DRX. When performinginter base station HO to cell 2, this mobile station becomes Active andperforms the HO operation.

Because the DRX level of the mobile station in the target cell in the HOcompletion period, is configured equal to the DRX level in the sourcecell in this example, the DRX level of the mobile station in cell 2 inthe HO completion period is determined Long DRX that is the same DRXlevel as that in cell 1.

In the HO completion period, base station 2 transmits a signal (EarlyDRX Control Signaling), which instructs the mobile station to transit toLong DRX, to the mobile station and starts the DRX control of the mobilestation immediately after the completion of HO.

The mobile station performs HO from cell 2 to cell 3 and from cell 3 tocell 4. In this case, as in the HO from cell 1 to cell 2, each basestation immediately instructs the mobile station to transit to Long DRXin the HO completion period and starts the DRX control. This reduces theextra power consumption of the mobile station in the cell after the HO.

FIG. 7 shows the DRX residence time of a mobile station during 30minutes on the assumption that the transition time from Active to DRX isone minute and the mobile station intermittently transmits and receivesdata during the 30 minutes. Because the time required for HO, several 10milliseconds, is so much shorter than the cell residence time that thistime is ignored in the calculation.

It is assumed that the frequency of the data transmission/reception ofthe mobile station is not so high in the HO destination that the mobilestation can stay in the DRX (the mobile station need not stay in theActive). It is also assumed that, in each cell, the mobile station movesthrough the center of the cell and moves the distance equal to thediameter of the cell. A model is assumed in which the cells are adjacenteach other and the mobile station moves on the straight line at auniform speed along the diameters of the multiple cells.

In case the present invention is used, the DRX level of the mobilestation is changed to a DRX level equal to that in the source level,immediately after the completion of the HO (for example, in severalmilliseconds).

For example, when the movement speed of the mobile station is 120 km/hand the diameter of the cell is 12 km in FIG. 7, the time the mobilestation resides in each cell is 6 minutes, HO is performed 4 times, andthe mobile station resides across 5 cells.

In the example shown in FIG. 13, the transition from Active to DRX istriggered by the time out in the timer which the base station has, asdescribed above, and hence the DRX residence time in 30 minutes is25(=(6−1)×5) minutes.

In contrast, because the mobile station can transit to the DRX in the HOcompletion period in this example, the DRX residence time during 30minutes is 29 minutes, which is the sum of 5(=6−1) minutes during whichthe mobile station resides in the first cell and 24 (=4×6) minutesduring which the mobile station resides in the cells after the HO.

As a result, the DRX residence period of the present invention is fourminutes longer than that (25 minutes) in the exemplary embodiment shownin FIG. 13. And, the power consumption of the mobile station can bedecreased in proportion to an increase in the DRX residence period.

On the other hand, when the movement speed is 60 km/h and the celldiameter is 1 Km, the mobile station resides in each cell for oneminute, HO is performed 29 times, and the mobile station resides across30 cells.

Because both the residence time in each cell and the transition timefrom Active to DRX are one minute in the example shown in FIG. 13, themobile station does not transit to DRX, during 30 minutes. That is, themobile station remains Active during 30 minutes.

In contrast, the mobile station can transit to DRX (for example, LongDRX), immediately after the completion of HO in this example and, ineach cell, the mobile station can stay in DRX after HO. For this reason,during 30 minutes, the mobile station can reside in DRX for the maximumof 29 minutes that is the sum of 0(=1−1) minute during which the mobilestation resides in the first cell and 29 (=1×29) minutes during whichthe mobile station reside in the cells after HO.

As a result, the DRX residence period according to the present inventionis 29 minutes longer than that in the case shown in FIG. 13, thusfurther decreasing the power consumption of the mobile station.

Second Exemplary Embodiment

FIG. 8 and FIG. 9 are diagrams showing a second exemplary embodiment ofthe present invention. As the second exemplary embodiment of the presentinvention, the following describes a case in which the current DRX levelL_(OLD) and the current DRX level residence time T are used as DormancyContext to determine the DRX level of the mobile station in the HOcompletion period in the target cell using expressions (13) and (14)(same as expressions (2) and (3)).L _(NEW) =L _(OLD) +M _(T)  (13)

$\begin{matrix}{M_{T} = \left\{ \begin{matrix}{M\; 1} & \left( {T \geq T_{0}} \right) \\{M\; 2} & \left( {T < T_{0}} \right)\end{matrix} \right.} & (14)\end{matrix}$

where T₀ is a predefined threshold, and M_(T) is a margin. Assume thatM1=−40% and M2=0% and that the value of the DRX level, if negative, isreplaced by 0.

In this example, the DRX level of the mobile station is called asfollows:

“Active” when the DRX level is 100%;

“Short DRX” when the DRX level is 60%;

“Long DRX” when the DRX level is 20%; and

“Idle” when the DRX level is 0%.

As described above, a DRX level lower than 100%, for example 90%, may bedefined as “Active”. For example, the ratio of the non-reception periodof the “Short DRX” in the DRX cycle is configured shorter than thenon-reception period of “Long DRX”.

Assume that the cells and base stations are related in such a way thatcells 1, 2, 3, and 4 are managed respectively by base stations 1, 2, 3,and 4 and that, in cells 2, 3, and 4, the mobile station performs datatransmission/reception only for the HO.

Assume that the change in the DRX level in this example is as shown inFIG. 8 when the mobile station performs inter base station HO.

In the initial state, assume that the mobile station that is moving fastresides in cell 1 and its DRX level is Long DRX. Also assume that thetime T₁, during which this mobile station stays in Long DRX in cell 1,is greater than or equal to T₀.

When performing an inter base station HO to cell 2, this mobile stationtransit to Active and performs the HO.

In this example, the DRX level of the mobile station in the target cellin the HO completion period is the value generated by adding the marginto the DRX level in the source cell. Therefore, the DRX level L_(NEW,2)of the mobile station in cell 2 in the HO completion period iscalculated by adding the margin M_(T)=M1=−40% to the DRX levelL_(OLD,1)=20% in cell 1. This calculation gives 0% (the actual value is−20%, and the negative value is replaced by 0) and determines that themobile station will transit to the Idle state.

In the HO completion period, base station 2 transmits a signal (EarlyDRX Control Signaling) to the mobile station to instruct it to transitto the Idle state.

The transition of the mobile station to the Idle state eliminates theneed for the mobile station to sequentially perform HO from cell 2 tocell 3 and from cell 3 to cell 4, thereby reducing the extra powerconsumption of the mobile station.

Another advantage is that the network (NW) can avoid an increase in theload that will be caused by repeated unnecessary HO of the mobilestation.

FIG. 9 shows the number of times the mobile station repeats HO until themobile station transits to Idle on the assumption that the transitiontime from Active to Long DRX is one minute and the transition time fromLong DRX to Idle is five minutes.

The time required for HO, several 10 m seconds, is so much shorter thanthe cell residence time that it is neglected in the calculation.

It is assumed that, in the initial state, the DRX level is Long DRX in acell where the mobile station resides first, the mobile stationdefinitely performs HO to the next cell, and the observation time is 30minutes.

It is also assumed that the mobile station moves through the center ofeach cell and moves the distance equal to the diameter. When the presentinvention is applied, the DRX level is determined to be the value,generated by adding the margin to the DRX level equal to that in thesource cell, immediately after the completion of HO (for example,several milliseconds).

In this example, if the maximum DRX residence time is less than fiveminutes, the mobile station stays in Long DRX and, if the maximum DRXresidence time is greater or equal to five minutes, a negative margin isadded to make the mobile station transit to Idle state (RRC_Idle).

In FIG. 9, when the movement velocity of the mobile station is 120 km/hand the cell diameter is 12 Km, the time the mobile station resides ineach cell is six minutes. The mobile station performs HO once only inthe first time and so the mobile station resides across two cells.

In the related art, the transition from Active to Long DRX or from LongDRX to Idle is triggered when the timer of the base station has timedout. Therefore, the mobile station transits to Long DRX in one minute inthe second cell and, after that, transits to Idle in five minutes.

In the present invention, because the mobile station transits to LongDRX in the HO completion period and, after that, transits to Idle infive minutes, HO is performed once as in the related art.

It takes one minute for the mobile station to transit to Long DRX in theexample shown in FIG. 13, whereas the mobile station in this exampletransits to Long DRX without waiting one minute but in as short asseveral milliseconds. This reduces the power consumption of the mobilestation.

Next, when the movement velocity is 60 Km/h and the cell diameter is 1km, the residence time in each cell is one minute.

Because the next HO is performed before the mobile station transits toLong DRX in the example shown in FIG. 13, the mobile station repeats HOduring 30 minutes of the observation time, with the result that HO isrepeated 29 times.

In contrast, because the mobile station is allowed to transit to LongDRX immediately after the first HO (for example, in severalmilliseconds) in this example, the Long DRX residence time is added up,even if HO is repeated. After HO is repeated five times, the mobilestation transits from Long DRX to Idle.

As a result, the present invention reduces the number of HO operationsby 24 as compared with the example shown in FIG. 13, reduces the powerconsumption of the mobile station, and reduces the load of the network(NW).

As described above, the present invention avoids extra power consumptionin the inter base station HO of a mobile station that performs the DRXoperation and an increase in the load of the NW.

In addition to those described above, the maximum transmission buffersize of the source base station, the average buffer size of the sourcecell base station, and so on may be used as Dormancy Context. Theaverage buffer size of the source cell base station and so on arecalculated either from the monitoring result of the transmission buffersize obtained via a periodical polling or from a log result of thetransmission buffer size based on the generation of an event at the timedata is accumulated in the transmission buffer.

The method described below may be used for performing the DRX control inthe target cell after HO (see FIG. 10).

When a mobile station and a base station have established the RRCconnection (RRC_Connected state) but the mobile station does not performdata transmission/reception for a predetermined time TD, the DRX levelof the mobile station is decreased as shown by expression (15).L _(NEW) =L _(OLD) −ΔL  (15)

Conversely, when a mobile station continues data transmission/receptionfor a predetermined time TU, the DRX level of the mobile station isincreased as shown by expression (16).L _(NEW) =L _(OLD) +ΔL  (16)

To implement this DRX control, one of the following two methods may beused.

-   -   The base station determines L_(NEW) and notifies the mobile        station of L_(NEW).    -   The base station notifies the mobile station of DL, TU, and TD,        and the base station and the mobile station each determine        L_(NEW).

This DRX control method is applicable not only to a mobile station thathas performed HO but also to the mobile stations residing in one cell.

Although the transition time from Active to Long DRX is assumed to takeone minute in the above example, the effect of the present inventionbecomes more remarkable as the transition time from Active to Long DRXbecomes longer. In the present invention, if a mobile station startscontinuous reception on the downlink but data is not transmitted for apredetermined period, there is a case in which the mobile station doesnot enter into DRX (discontinuous reception), but transits to theRRC_IDLE state.

FIG. 14 is a diagram schematically showing an example of theconfiguration of a base station in the example shown in FIG. 1 and FIG.2. Because the source base station (101) and the target base station(102) in FIG. 1 and FIG. 2 have the same configuration, FIG. 14 showsthe configuration of the source base station only. Referring to FIG. 14,the source base station comprises a radio unit (RF) 105 that has atransmission unit and a reception unit not shown, a baseband unit 106that performs baseband processing, a coding/decoding unit (CODEC) 107that codes transmission data and decodes reception data, a control unit108, a transmission/reception unit 109 that communicates with a targetbase station via a wired line, a DRX controller 110 that derives a DRXlevel, a buffer unit 111, and a coding/decoding unit 112 that codes acontrol signal to be transmitted and decodes a received control signal.

The control unit 108 comprises a scheduler 108-1 that controls theoperation of the coding/decoding unit (CODEC) 107 and the DRX controller110 and a controller 108-2 that controls the transmission/reception unit109. The buffer unit 111 comprises a transmission buffer (not shown) inwhich transmission data is accumulated and a reception buffer (notshown) in which reception data is accumulated. The DRX controller 110monitors data accumulated in the transmission buffer of the buffer unit110, derives the Activity level of a mobile station and, as describedabove, derives a DRX level having the correlation with the Activitylevel itself or with the Activity level obtained from the operation forthe Activity level. The scheduler 108-1 notifies the DRX controller 110when to monitor the transmission buffer.

When a DRX level is acquired from the DRX controller 110, the controller108-2 performs the DRX to transmit the signal (DRX Control Signaling) tothe mobile station. The control signal from the control unit 108 iscoded by the coding/decoding unit 112 to generate a control signalcorresponding to DRX Control Signaling and, after the basebandprocessing is performed, the control signal is radio-transmitted to themobile station from the radio unit 105. The controller 108-2 transmitsnot only Dormancy Context including the DRX level received from the DRXcontroller 110 but also Context Data including QoS Profile and AsConfiguration to the target base station via the transmission/receptionunit 109. In addition, when a signal (Context Confirm, HO Completed,etc.) is received from the target base station via thetransmission/reception unit 109, the controller 108-2 notifies thescheduler 108-1 of the received signal and, when the corresponding eventis generated, the scheduler 108-1 schedules the next processing.

In the present invention, a 3GPP-LTE portable terminal may be used as amobile station. As described above, either the base station side maydetect the Activity level of a mobile station and derive the DRX levelor the mobile station side detects the Activity level of the mobilestation and notifies the base station of the detected Activity level.FIG. 15 is a diagram showing an example of the configuration of a mobilestation in one example of a communication terminal of the presentinvention. Referring to FIG. 15, an Activity level controller (ActivityLevel CTRL) 126 of the mobile station (communication terminal) 103monitors the accumulation state of the transmission buffer of a bufferunit 124 and calculates the Activity level. A control unit 125, whichcomprises a schedule unit not shown, controls the monitoring of theaccumulation state of the transmission buffer of the buffer unit 124.The Activity level may be transmitted to the base station, for example,as the control signal to allow the base station to derive the DRX levelbased on the Activity level, received from the mobile station, and toperform the DRX control. In the non-reception period of the DRX cycle,the mobile station 103 inactivates the RF reception unit (not shown) ofan RF unit 121. The description of a baseband unit 122, CODEC units 123and 127, etc., is omitted.

FIG. 16 is a diagram showing an example of the configuration of a mobilestation in another example of a communication terminal of the presentinvention. The mobile station (communication terminal) in this examplecomprises a DRX level controller (DRX Level CTRL) 128 instead of theActivity level controller shown in FIG. 15. The DRX level controller 128monitors the accumulation state of the transmission buffer (not shown)of the buffer unit 124 to calculate the Activity level and derives theDRX level according to the Activity level. The mobile stationautonomously performs the DRX control according to the acquired DRXlevel. When transitioning to the DRX control, the mobile stationtransmits a control signal to the base station to notify it about theDRX level and the start of the DRX control, and the base station recordsand manages the start of the DRX control.

Next, as another example of the present invention, the followingdescribes an example of a mobile station compatible with the dualoperation of 3GPP LTE and WCDMA (Wideband Code Division MultipleAccess). FIGS. 17A and 17B are diagrams schematically showing anotherexample of the present invention. FIG. 17B is a diagram showing theconfiguration of the dormancy control unit of a base station controlstation (RNC: also called “Radio Network Controller”) 4 shown in FIG.17A. At least Dormancy Context is transferred from a first LTE basestation 1 to a second LTE base station 2 to allow the second LTE basestation 2 to immediately perform the DRX control according to the DRXlevel that has been used by the first LTE base station 1. When handoverfrom the second LTE base station 2 to a WCDMA base station 5 isperformed, at least Dormancy Context is transferred from the second LTEbase station 2 to the base station control station (RNC) 4 and the DRXlevel is transmitted from the base station control station 4 to the basestation 5 to allow the WCDMA base station 5 to perform the DRX controlof a mobile station 3 according to the mobile station activity state ofthat 3GPP-LTE mobile station before the handover. As shown in FIG. 17B,the base station control station 4 comprises a Dormancy Control relayunit 44 that receives Dormancy Context from the LTE base station via atransmission/reception interface 41 and transmits it to the WCDMA basestation 5, which is under the base station control station 4, via atransmission/reception interface 42.

It is, as a matter of course, possible to apply the present invention tohandover between WLAN (Wireless Local Area Network) access points (AP)and to handover between WiMAX (Wireless interoperability of MicrowaveAccess) base stations.

The present invention is applicable also to the control of thediscontinuous reception of a first node when a transition occurs from astate, in which the first node and a second node that can performradio-communication with each other are relatively movable and thesecond node manages the first node, to a state in which the first nodeand a third node that can perform radio-communication with each other(the third node can communicate with the second node) are relativelymovable and the third node manages the first node.

FIG. 19 is a diagram showing a flow of inter base station handover in amodification of one exemplary embodiment of the present invention. Inthe exemplary embodiment shown in FIG. 1, the target base station 102transmits the handover completion signal (HO Completed) to the sourcebase station 101 and, after that, transmits the DRX control startinstruction signal (Early DRX Control Signaling) to the mobile station103. Instead of transmitting this signal (Early DRX Control Signaling),it is also possible to transmit the DRX control information to themobile station 103, as shown in FIG. 19, by including DRX controlinformation (for example, contents equivalent to the contents of EarlyDRX Control Signaling such as DRX level or DRX cycle) into a signal(Context Confirm), transmitted from the target base station 102 to thesource base station 101, and into a command (HO Command) transmittedfrom the source base station 101 to the mobile station 103. That is,when Context Data is received from the source base station 101 in FIG.19, the target base station 102 executes the DRX selection processing(DRX Selection) based on Dormancy Context included in Context Data andtransmits the selected DRX control information (New DRX controlinformation) to the source base station 101 via the signal (ContextConfirm). The source base station 101 transmits the DRX controlinformation (New DRX control information) to the mobile station 103 viaa command (HO Command). The mobile station 103, which has received thesignal (Context Confirm) transmits a signal (HO Confirm) to the targetbase station 102 and, immediately after the response indicating that asignal (HO Confirm) is correctly received is returned from the targetbase station 102, starts DRX.

To enable the proper consumption of the mobile station battery, DRX inE-UTRAN (Evolved UTRAN) has the following features.

There is no sub-state of RRC and MAC (Medium Access Control) fordistinguishing between different DRX levels.

DRX values that can be utilized are controlled by the network (NW) andare present for x seconds from the non-DRX state. The value x may be thesame as paging DRX used in LTE_IDLE (Actual values will be studied infuture and are not defined in this specification).

The measurement request and the reporting criterion may vary accordingto the length of the DRX period. That is, a longer DRX period maycorrespond to a more relinquished request.

When the radio quality of the serving (serving cell) (accuratedefinition of radio quality will be studied in future) is higher than athreshold, the network (NW) may transmit the threshold to the mobilestation (UE) to indicate that there is no need for the measurement ofneighboring cells.

Regardless of the DRX cycle, a mobile station (UE) may use a firstavailable opportunity of RACH in order to transmit a measurement report(UL measurement report). Immediately after transmitting the measurementresult, the mobile station (UE) may change its own DRX operation(whether or not the method is predefined by the eNB will be studied infuture).

HARQ processing regarding uplink data transmission is independent of theDRX processing. Whether or not the DL data HARQ processing isindependent of the DRX processing will be studied in future.

During handover, a source eNB transfers Dormancy Context to a target eNBto optimize the continuation of DRX control before and after thehandover. The Dormancy Context includes at least the latest DRX leveland an average/maximum/minimum DRX level in the source cell. If the UEhas been at a low DRX level in the source cell, the target eNB may useDormancy Context also for the processing for shifting the state of theUE to LTE_IDLE.

While the present invention has been described with reference to theexamples above, it is to be understood that the present invention is notlimited to the configuration of the examples above and thatmodifications and changes that may be made by those skilled in the artwithin the scope of the present invention are included.

The exemplary embodiments and the examples may be changed and adjustedin the scope of all disclosures (including claims) of the presentinvention and based on the basic technological concept thereof. In thescope of the claims of the present invention, various disclosed elementsmay be combined and selected in a variety of ways.

The invention claimed is:
 1. A base station which is a first basestation of a plurality of base stations in a wireless communicationsystem, the first base station comprising: a transmission unit whichtransmits, when a mobile station performs a handover from the first basestation to a second base station, information to the second basestation, wherein the information comprises first Discontinuous Reception(DRX) configuration information and time that the mobile station staysin RRC Connected state at the first base station; and a reception unitwhich receives, from the second base station, second DRX configurationinformation in response to receiving the first DRX configurationinformation at the second base station from the first base station.
 2. Amethod of controlling a base station which is a first base station of aplurality of base stations in a wireless communication system, the firstbase station comprising: transmitting, when a mobile station performs ahandover from the first base station to a second base station,information to the second base station, wherein the informationcomprises first Discontinuous Reception (DRX) configuration informationand time that the mobile station stays in RRC Connected state at thefirst base station; and receiving, from the second base station, secondDRX configuration information in response to receiving the first DRXconfiguration information at the second base station from the first basestation.
 3. A mobile station in a wireless communication systemincluding a plurality of base stations, comprising: a control unit whichperforms a handover from a first base station to a second base station,wherein the first base station transmits first Discontinuous Reception(DRX) configuration information and time that the mobile station staysin RRC Connected state at the first base station to the second basestation; and a radio unit which receives a second DRX configurationinformation via the first base station, wherein the second base stationtransmits the second DRX configuration information to the first basestation in response to receiving the first DRX configurationinformation.
 4. A method of controlling a mobile station in a wirelesscommunication system including a plurality of base stations, comprising:performing a handover from a first base station to a second basestation, wherein the first base station transmits first DiscontinuousReception (DRX) configuration information and time that the mobilestation stays in RRC Connected state at the first base station to thesecond base station; and receiving a second DRX configurationinformation via the first base station, wherein the second base stationtransmits the second DRX configuration information to the first basestation in response to receiving the first DRX configurationinformation.